1. Field of the Invention
The present invention relates to a developing apparatus to be used for developing an electrostatic latent image formed on an electrostatic latent image-bearing member such as a photosensitive member or an electrostatic recording derivative, and an electrophotographic image-forming apparatus including the developing apparatus.
2. Description of the Related Art
Electrophotography generally involves the utilization of a photoconductive substance, and includes: forming an electrostatic latent image on an electrostatic latent image-bearing member (photosensitive drum) by various means; applying a developing bias to a developing zone; developing the electrostatic latent image with a developer to form a toner image; transferring the toner image onto a transfer material such as paper as required; and fixing the toner image on the transfer material with heat or pressure to provide a copy. Developing methods in the electrophotography are mainly classified into a one-component developing method in which there is no need for a carrier and a two-component developing method involving the use of a carrier. A developing apparatus employing the one-component developing method is advantageous in that since no carrier is needed, the frequency at which toner must be exchanged owing to deterioration in the toner can be reduced; in addition, there is no need to provide the developing apparatus with, for example, a mechanism for adjusting the concentration of the toner or carrier, so the developing apparatus itself can be reduced in size and weight.
Japanese Patent Application Laid-Open No. 2005-157318 discloses that the particle size of a developer (toner) is reduced and the saturation magnetization of the developer is reduced in order that the image quality of a copy may be higher.
However, when the amount of a magnetic material is reduced and the particle size of a developer is reduced, a so-called charge-up phenomenon as described below is apt to occur: the developer is brought into a passive state by mirror image force with the surface of a developing sleeve, so a latent image on a photosensitive drum is difficult to develop with the developer from the developing sleeve. As a result, a reduction in image density may occur.
To deal with the charge-up of a developer, Japanese Patent Application Laid-Open No. 2003-323042 proposes a developer bearing member having a resin layer which is incorporated with graphitized particles having a degree of graphitization p(002) of 0.20 to 0.95 and an indentation hardness HUT[68] of 15 to 60 are incorporated. The charge-up of the developer is alleviated by the effect of the graphitized particles of enhancing the performance of rapidly and stably charging the developer.
However, according to investigation made by the inventors of the present invention, when electrophotographic images are formed from a one-component, magnetic toner having a small particle diameter and a small saturation magnetization according to a predetermined printing mode, the following phenomenon occurs: an image density after rest largely fluctuates as compared with that before the pause as shown in FIG. 6. The term “predetermined printing mode” as used herein refers to the following printing condition: after 1,000 or more sheets are continuously printed, a pause period of 30 minutes to 2 hours is set, and then 1,000 or more sheets are printed again. The inventors have found that, when electrophotographic images are formed according to the printing mode, an image density on the first sheet after the rest is extremely higher than an image density before the pause. In addition, the inventors have found that an image density gradually returns to the image density before the pause by continuously performing image formation after the pause.